The present invention relates to a method and an arrangement for transferring data or voice via radio between two nodes in a mobile radio system.
In public land mobile networks, PLMNs, the geographic area to which the network provides services, such as voice and data traffic, is divided into cells. A cell, in this context, is a limited geographic area to which these services are provided by a certain base transceiver station, BTS, belonging to the network.
The size of the cell is normally set depending on the traffic density in the area in question. In most systems it is possible to provide a small cell within a larger cell, meaning that a subscriber station residing in the smaller cell may communicate with both the BTS associated with the smaller cell and the BTS associated with the larger cell. This allows the system to provide extra system capacity to so called xe2x80x9chot spotsxe2x80x9d where the traffic density is high. These BTSs should then utilise different frequency bands.
A clear trend in mobile telephony system is an increase in the number of small and very small cells, sometimes named xe2x80x9cmicrocellsxe2x80x9d or even xe2x80x9cpicocellsxe2x80x9d. This results in more flexible systems with an overall increased system capacity.
When a new BTS is introduced it must be connected to the next hierarchical level of the PLMN, which in the GSM case is a base station controller, a BSC. Normally this is arranged by means of a wired connection, i.e. a cable. When the number of small cells increases, however, this approach becomes expensive and cumbersome. A feasible method to overcome this problem is to use a radio link to provide contact between the master node, the BSC and the slave node, the BTS. It should be noted that each master node may be associated with a number of slave nodes. This, however, requires relatively high powered radio transmitters and accurate frequency planning. It also requires sophisticated receivers capable of compensating for multipath fading. This may be accomplished by means of rake-receivers or equalisers. The cost of such equipment is normally substantially higher than the costs of a wired connection.
Another approach for connecting a BSC to a BTS is to provide a regular microwave link between the two nodes. This may then be performed with directional antennas at both nodes accomplishing up- and downlinks in narrow beams between them. This may be performed with relatively low powered transmitters and simple receivers and does not normally require any complicated frequency planning. For this approach to be possible, however, it is necessary to have a line of sight between the respective antennas. This often difficult to accomplish in densely populated areas. Moreover, if the line of sight is disturbed by a temporarily shadowing object the communication between the nodes is shut down.
In mobile systems, and especially in so called point to multipoint, P-MP systems designed for transmission with high requirements on radio-link performance, multipath propagation is quite destructive for such performance. This is particularly the case when no line of sight exists between transmitter and receiver due to obstacles. As a measure of link performance, bit error rate, BER may be used, where BER≈xc2xd*square(S/T), where S is rms delay and T is transmitted symbol duration. A normal way to encounter this problem is to introduce equalisers in TDMA systems and so called RAKE-receivers CDMA systems and to combine these features with different kinds of coding. In high bitrate systems equalisers and RAKE receivers are expensive and cumbersome to implement. They also introduce delay into the system due to base band processing.
One object of the present invention is therefore to provide a method for accomplishing a flexible and inexpensive connection between two, preferably fixed, nodes in a mobile telephony system, where the connection carries data or voice traffic which is sensitive to delay due to multipath propagation and/or fading.
This object is fulfilled by a method for transferring data or voice traffic between first and second fixed nodes in a mobile radio system, where each node includes an adaptive phase controlled antenna unit. A first link is established where the first node transmits radio signals to the second node in a narrow beam deliberately via a reflecting body. A second link is established where the second node transmits radio signals to the first the node in a narrow beam deliberately via a reflecting body. The direction of transmission of the second link is then the direction of reception of the signal received from the first link. This method results in a flexible communication link between the nodes that functions properly even if there is no line of sight between the two nodes. The transmitters may be relatively low powered and the receivers need not compensate for multipath fading. The connection is flexible and may be adjusted as a result of a changing electrical environment.
In an embodiment of the invention the first and second links are established in a common radio frequency band. This frequency band is then shared by using time division duplex, TDD. This allows the links to be set up using a more limited frequency band, no guard band is required between duplex bands.
In another embodiment of the invention the first and second links are established in different radio frequency bands, using frequency division duplex, FDD. Then the duplex distance between the links should be less than 2 percent. This allows the up- and downlinks to be established along the same path (albeit in different directions) and results in less expensive transmitters and receivers.
Another object of the present invention is to provide an arrangement for accomplishing a flexible and inexpensive connection between two, preferably fixed, nodes in a mobile telephony system.
This object is fulfilled by an arrangement for transferring data or voice between first and second nodes in a mobile radio system, where each node includes a phase controlled antenna unit. The arrangement then includes means for establishing a first radio link is where the first node transmits radio signals to the second node in a narrow beam deliberately via a reflecting body. It furthermore includes means for establishing a second radio link where the second node transmits radio signals to the first node in a narrow beam deliberately via a reflecting body. The direction of transmission of the second link is then the direction of reception of the first link. This results in a flexible communication link between the nodes that functions properly even if there is no line of sight between the two nodes. The transmitters may be relatively low powered and the receivers need not compensate for multipath fading.
In a preferred embodiment the first node is connected to a base station controller and that the second node is connected to a base transceiver station. This allows the invention to be used in order to simplify cell planning activities.